1. Field of the Invention
The present invention relates to a vehicle height adjustment method, and more particularly to a method of operating an apparatus known as a vehicle levelling system, for adjusting the height of a vehicle by supplying a working fluid to or discharging the working fluid from dampers mounted on front and rear portions of the vehicle.
2. Description of the Prior Art
Vehicle height adjusting apparatus serve to keep the height of a vehicle at a reference level at all times through adjustment of the vehicle height irrespectively of a variation in the load on the vehicle. The vehicle height adjusting apparatus maintains the vehicle height at a proper level with respect to the road surface and also varies the suspension capability of the vehicle wheel suspension to give the driver and other passengers good riding comfort. The vehicle height adjusting apparatus generally comprises a pump unit for supplying a working fluid under pressure, such as pressurized air, dampers mounted respectively on wheel suspension units and contractible or extensible to vary the height of the vehicle when the working fluid is discharged from or supplied to the dampers, a fluid system including inlet and outlet ports and valves, and an electric circuit for controlling operation of the pump unit and fluid system. With the conventional vehicle height adjusting apparatus, the working fluid having been used for height adjustment, generally remains under high pressure in a closed space in the fluid passage leading from an outlet port of the pump unit. When the pump unit is started by an electric motor while the fluid is trapped under high pressure in the pump outlet port, the pump unit and the electric motor are subjected to a large load. To avoid this, the electric motor for driving the pump unit is required to be of a large size and have a large capacity. However, electric motors for use on vehicles such as passenger cars fail to meet such a requirement because of limited space and available rigidity of the cars. Another problem is that a large current is required for getting the motor started when it is under a large load, but the pump cannot be started at the time the voltage of the car battery is lowered and no sufficient starting current is available for the motor.
To avoid the problem of the large load imposed on the pump unit and the motor when the working fluid is to be supplied, it has been practiced to open the inlet valve of the pump unit to thereby reduce the load applied thereto. However, such a practice requires a change in the construction and function of the pump unit, and is costly to perform. Furthermore, it is troublesome to change the motor construction and function.
Another proposal has been to reduce the volumetric efficiency of the pump unit for reducing the load applied to the motor when the pump unit is to be started. This is disadvantageous in that the time required for vehicle height adjustment is prolonged, and desired vehicle height adjustment cannot be completed by the time it is needed.
The vehicle height adjusting apparatus of the conventional construction has another difficulty besides the foregoing problems. Where the pump unit is disposed on a front portion of the vehicle, fluid passages connected to the dampers for the rear wheels are longer than the fluid passages joined to the dampers for the front wheels. When the pump unit is actuated and solenoid-operated valves are simultaneously opened to supply the working fluid from the pump unit to the front and rear wheels in order to increase the vehicle height, the fluid flow to the rear wheel dampers is subjected to a larger resistance due to the longer fluid passages and reduced cross-sectional areas thereof selected for increased durability. As a result, the fluid pressures in the front and rear wheel dampers become different, that is, the fluid pressure in the rear wheel dampers is lower than that in the front wheel dampers. The vehicle height as thus adjusted is improper in that the front and rear portions of the vehicle have different heights.
When the rear wheels are subjected to a larger load than the front wheels, for example, and vehicle height adjustment is performed to keep the front and rear vehicle portions at the same level, the fluid pressure in the rear wheel dampers becomes higher than that in the front wheel dampers. Then, the load on the rear wheels is reduced, and the solenoid-operated valves for discharging the fluid from the dampers are simultaneously opened to lower the vehicle height. At this time, the fluid flows under high pressure from the rear wheel dampers to the front wheel dampers, with the result that the vehicle height at the front wheels is temporarily increased.
The present invention effectively eliminates the foregoing shortcomings attendant the conventional apparatus.